I haven't really gone out of my way monster-wise, but this is what I've found so far:

The metal cactuar is a good Commando. It gobbles your best materials, but it's worth considering that the cactuars tend to have unique abilities that are useful. Definitely pick up a Flanitor for healing. It starts out weak but once you get it maxed to level 40 it has a magic stat of over 700, and it's special ability fully heals your party. My current ravager is Apotamkin. Nice stats, an equal 700-ish for both magic and strength, HP isn't too bad. Has the item collector ability, and I made it gobble a goblin thing for gil finder. Picked up a Gigantuar and it looks like it might be the best Synergist of everything I've seen so far. Early Peakers are good to max out as soon as you find them. They usually have high stats for where you are story-wise.

Does anyone know if Cait Sith is worth boosting to the end? Or any of those late bloomers for that matter? I've been keeping them and boosting them every now and then, but they're on the back burner so none of them are really past their 30s yet.